This invention relates generally to the development of latent electrostatic images on a moving electrographic recording medium, and in particular to an improved development electrode array for assisting the deposition of electrostatically charged toner particles on the charge retaining surface of the moving recording medium.
In one type of electrophotographic recording process, the recording medium is a film having a polyester base, a transparent conductive layer that overlies the polyester base and a photoconductive layer that overlies the transparent conductive layer. To record on the film, a uniform electrostatic charge is first applied, by means of a charging corona element, to the surface of the photoconductive layer of the film. The uniform charge is then selectively dissipated by exposing the surface to a light image of the pattern to be recorded. The resulting pattern of charges is an electrostatic latent image on the surface of the photoconductive layer which can then be rendered visible, for example, in an attraction toning process, by applying liquid toner containing electrostatically charged developer particles in suspension to the surface of the photoconductive layer. The charged developer particles are attracted and held to the surface of the photoconductive layer by the electrostatic force developed between the developer particles and the charge retained on the surface of the photoconductive layer. A permanent visible image can be obtained, for example, by using developer particles which can be heat fused to the photoconductive layer, and then subjecting the visible image to a heat fusing step.
It is well known in the electrophotographic art that the development process is improved when a development electrode is used to assist the deposition of electrostatically charged particles on the charge retaining surface of the recording medium. As disclosed at page 28 of Electrophotography by R. M. Schaffert, published by the Focal Press, 1966, it is common to apply a suitable positive or negative voltage to the development electrode to compensate for incorrect exposure, to alter the tonal characteristics of a picture or to accomplish reversal development.
As disclosed in U.S. Pat. No. 3,964,436, issued to John D. Plumadore, and assigned to the assignee of this invention, electrophotographic recording systems can have variations in the imaging-light intensity, variations in the degree of darkness of documents being copied, and variations in the properties of the photoconductors used in the recording medium. These variations can result in the electrostatic charge potentials which remain on the film after imaging being higher or lower than desired, which in turn affects the attractive forces upon the toner particles during development. Of particular importance is the difference between the highest and lowest charge potential on the imaged film. Where this difference is very slight, there is a tendency to produce dark, or unclean, background areas in the image. Additionally, where the lowest charge potential on the imaged film is greater in magnitude than zero, there is a tendency for the toner particles to also deposit in the areas having that lowest charge potential. In the preferred type of electrophotographic recording apparatus, the background of the documents being copied usually corresponds to the lowest charge potential on the film, and that area on the film is seldom discharged to zero, due to photoconductor properties and/or practical limitations on imaging-light duration and intensity.
As disclosed in the Plumadore U.S. Pat. No. 3,964,436, the variations in the electrostatic charge potentials on the imaged film can be handled by a large area development electrode that is electrically floating and positioned parallel to and close to the area of the film being developed during application of toner particles to the frame. The large area development electrode has induced upon it an electrostatic potential between the highest and lowest charge potential on the area of the film being developed.
Besides the variations discussed in the aforementioned Plumadore patent, in a given image area there can be a variation in the charge potential on the recording medium that corresponds to the background area of the image being recorded. For example, the optical system used to expose the charged conductive layer of the medium will usually illuminate the central portion of the image more effectively than the peripheral portions of the image. Thus, for a given image background, a lower charge potential will occur at the central portion of the image area than at the peripheral portions of the image area. In order to prevent toning of the background areas, the potential of the development electrode must exceed the charge potential on the recording medium that corresponds to the background area. When a large area floating development electrode as described in the previously discussed Plumadore patent is employed wherein the floating development electrode subtends the entire image area being developed, the floating development electrode will have induced on it a potential approximately equal to the average potential of the entire imaged area. For images that contain a very high percentage of background area, the potential induced on the development electrode can approach or even become less than the charge potential level that corresponds to the background areas in the peripheral portion of the image. If that occurs, some toner particles, in an attraction toning process, will be deposited on the peripheral background areas.
One electrophotographic recording system that employs plural floating electrodes is disclosed in U.S. Pat. No. 3,892,481, issued to L. F. Schafer et al. However, the floating electrodes 62, 64, 66 of that system are not employed as development electrodes. The system described in Schafer et al does employ a plurality of development electrodes 72, 74, 76, 78, each development electrode being connected to a voltage source. The floating electrodes are used to sense the charge potential of three areas across the width of the recording medium. A circuit selects the floating electrode having the lowest induced voltage to control the voltage level applied to the development electrodes.
It is an object of this invention to provide an improved development electrode arrangement for assisting the deposition of charged toner particles to the charge retaining surface of a moving electrographic recording medium.
It is another object of this invention to provide an improved floating electrode arrangement for assisting the development of a latent electrostatic image on a moving electrographic recording medium.
And yet another object of this invention is to provide a developing electrode arrangement that is able to adjust to varying background charge potential levels that may exist in different portions of the image area being developed.